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loop


Loop

Scan

  • A general form of recurrence, which can be used for looping.
  • Reduction and map (loop over the leading dimensions) are special cases of scan.
  • You scan a function along some input sequence, producing an output at each time-step.
  • The function can see the previous K time-steps of your function.
  • sum() could be computed by scanning the z + x(i) function over a list, given an initial state of z=0.
  • Often a for loop can be expressed as a scan() operation, and scan is the closest that Theano comes to looping.
  • Advantages of using scan over for loops:
    • Number of iterations to be part of the symbolic graph.
    • Minimizes GPU transfers (if GPU is involved).
    • Computes gradients through sequential steps.
    • Slightly faster than using a for loop in Python with a compiled Theano function.
    • Can lower the overall memory usage by detecting the actual amount of memory needed.

The full documentation can be found in the library: Scan.

Scan Example: Computing tanh(x(t).dot(W) + b) elementwise

import theano
import theano.tensor as T
import numpy as np

# defining the tensor variables
X = T.matrix("X")
W = T.matrix("W")
b_sym = T.vector("b_sym")

results, updates = theano.scan(lambda v: T.tanh(T.dot(v, W) + b_sym), sequences=X)
compute_elementwise = theano.function(inputs=[X, W, b_sym], outputs=[results])

# test values
x = np.eye(2, dtype=theano.config.floatX)
w = np.ones((2, 2), dtype=theano.config.floatX)
b = np.ones((2), dtype=theano.config.floatX)
b[1] = 2

print(compute_elementwise(x, w, b)[0])

# comparison with numpy
print(np.tanh(x.dot(w) + b))
[[ 0.96402758  0.99505475]
 [ 0.96402758  0.99505475]]
[[ 0.96402758  0.99505475]
 [ 0.96402758  0.99505475]]

Scan Example: Computing the sequence x(t) = tanh(x(t - 1).dot(W) + y(t).dot(U) + p(T - t).dot(V))

import theano
import theano.tensor as T
import numpy as np

# define tensor variables
X = T.vector("X")
W = T.matrix("W")
b_sym = T.vector("b_sym")
U = T.matrix("U")
Y = T.matrix("Y")
V = T.matrix("V")
P = T.matrix("P")

results, updates = theano.scan(lambda y, p, x_tm1: 
T.tanh(T.dot(x_tm1, W) + T.dot(y, U) + T.dot(p, V)), sequences=[Y, P[::-1]],
outputs_info=[X]) compute_seq = theano.function(inputs=[X, W, Y, U, P, V], outputs=[results])
# test values x = np.zeros((2), dtype=theano.config.floatX) x[1] = 1 w = np.ones((2, 2), dtype=theano.config.floatX) y = np.ones((5, 2), dtype=theano.config.floatX) y[0, :] = -3 u = np.ones((2, 2), dtype=theano.config.floatX) p = np.ones((5, 2), dtype=theano.config.floatX) p[0, :] = 3 v = np.ones((2, 2), dtype=theano.config.floatX) print(compute_seq(x, w, y, u, p, v)[0]) # comparison with numpy x_res = np.zeros((5, 2), dtype=theano.config.floatX) x_res[0] = np.tanh(x.dot(w) + y[0].dot(u) + p[4].dot(v)) for i in range(1, 5): x_res[i] = np.tanh(x_res[i - 1].dot(w) + y[i].dot(u) + p[4-i].dot(v)) print(x_res)
[[-0.99505475 -0.99505475]
 [ 0.96471973  0.96471973]
 [ 0.99998585  0.99998585]
 [ 0.99998771  0.99998771]
 [ 1.          1.        ]]
[[-0.99505475 -0.99505475]
 [ 0.96471973  0.96471973]
 [ 0.99998585  0.99998585]
 [ 0.99998771  0.99998771]
 [ 1.          1.        ]]

Scan Example: Computing norms of lines of X

import theano
import theano.tensor as T
import numpy as np

# define tensor variable
X = T.matrix("X")
results, updates = theano.scan(lambda x_i: T.sqrt((x_i ** 2).sum()), sequences=[X])
compute_norm_lines = theano.function(inputs=[X], outputs=[results])

# test value
x = np.diag(np.arange(1, 6, dtype=theano.config.floatX), 1)
print(compute_norm_lines(x)[0])

# comparison with numpy
print(np.sqrt((x ** 2).sum(1)))
[ 1.  2.  3.  4.  5.  0.]
[ 1.  2.  3.  4.  5.  0.]

Scan Example: Computing norms of columns of X

import theano
import theano.tensor as T
import numpy as np

# define tensor variable
X = T.matrix("X")
results, updates = theano.scan(lambda x_i: T.sqrt((x_i ** 2).sum()), sequences=[X.T])
compute_norm_cols = theano.function(inputs=[X], outputs=[results])

# test value
x = np.diag(np.arange(1, 6, dtype=theano.config.floatX), 1)
print(compute_norm_cols(x)[0])

# comparison with numpy
print(np.sqrt((x ** 2).sum(0)))
[ 0.  1.  2.  3.  4.  5.]
[ 0.  1.  2.  3.  4.  5.]

Scan Example: Computing trace of X

import theano
import theano.tensor as T
import numpy as np
floatX = "float32"

# define tensor variable
X = T.matrix("X")
results, updates = theano.scan(lambda i, j, t_f: T.cast(X[i, j] + t_f, floatX),
                  sequences=[T.arange(X.shape[0]), T.arange(X.shape[1])],
                  outputs_info=np.asarray(0., dtype=floatX))
result = results[-1]
compute_trace = theano.function(inputs=[X], outputs=[result])

# test value
x = np.eye(5, dtype=theano.config.floatX)
x[0] = np.arange(5, dtype=theano.config.floatX)
print(compute_trace(x)[0])

# comparison with numpy
print(np.diagonal(x).sum())
4.0
4.0

Scan Example: Computing the sequence x(t) = x(t - 2).dot(U) + x(t - 1).dot(V) + tanh(x(t - 1).dot(W) + b)

import theano
import theano.tensor as T
import numpy as np

# define tensor variables
X = T.matrix("X")
W = T.matrix("W")
b_sym = T.vector("b_sym")
U = T.matrix("U")
V = T.matrix("V")
n_sym = T.iscalar("n_sym")

results, updates = theano.scan(lambda x_tm2, x_tm1: T.dot(x_tm2, U) + T.dot(x_tm1, V) + T.tanh(T.dot(x_tm1, W) + b_sym),
                    n_steps=n_sym, outputs_info=[dict(initial=X, taps=[-2, -1])])
compute_seq2 = theano.function(inputs=[X, U, V, W, b_sym, n_sym], outputs=[results])

# test values
x = np.zeros((2, 2), dtype=theano.config.floatX) # the initial value must be able to return x[-2]
x[1, 1] = 1
w = 0.5 * np.ones((2, 2), dtype=theano.config.floatX)
u = 0.5 * (np.ones((2, 2), dtype=theano.config.floatX) - np.eye(2, dtype=theano.config.floatX))
v = 0.5 * np.ones((2, 2), dtype=theano.config.floatX)
n = 10
b = np.ones((2), dtype=theano.config.floatX)

print(compute_seq2(x, u, v, w, b, n))

# comparison with numpy
x_res = np.zeros((10, 2))
x_res[0] = x[0].dot(u) + x[1].dot(v) + np.tanh(x[1].dot(w) + b)
x_res[1] = x[1].dot(u) + x_res[0].dot(v) + np.tanh(x_res[0].dot(w) + b)
x_res[2] = x_res[0].dot(u) + x_res[1].dot(v) + np.tanh(x_res[1].dot(w) + b)
for i in range(2, 10):
  x_res[i] = (x_res[i - 2].dot(u) + x_res[i - 1].dot(v) +
              np.tanh(x_res[i - 1].dot(w) + b))
print(x_res)
[array([[  1.40514825,   1.40514825],
       [  2.88898899,   2.38898899],
       [  4.34018291,   4.34018291],
       [  6.53463142,   6.78463142],
       [  9.82972243,   9.82972243],
       [ 14.22203814,  14.09703814],
       [ 20.07439936,  20.07439936],
       [ 28.12291843,  28.18541843],
       [ 39.1913681 ,  39.1913681 ],
       [ 54.28407732,  54.25282732]])]
[[  1.40514825   1.40514825]
 [  2.88898899   2.38898899]
 [  4.34018291   4.34018291]
 [  6.53463142   6.78463142]
 [  9.82972243   9.82972243]
 [ 14.22203814  14.09703814]
 [ 20.07439936  20.07439936]
 [ 28.12291843  28.18541843]
 [ 39.1913681   39.1913681 ]
 [ 54.28407732  54.25282732]]

Scan Example: Computing the Jacobian of y = tanh(v.dot(A)) wrt x

import theano
import theano.tensor as T
import numpy as np

# define tensor variables
v = T.vector()
A = T.matrix()
y = T.tanh(T.dot(v, A))
results, updates = theano.scan(lambda i: T.grad(y[i], v), sequences=[T.arange(y.shape[0])])
compute_jac_t = theano.function([A, v], [results], allow_input_downcast=True) # shape (d_out, d_in)

# test values
x = np.eye(5, dtype=theano.config.floatX)[0]
w = np.eye(5, 3, dtype=theano.config.floatX)
w[2] = np.ones((3), dtype=theano.config.floatX)
print compute_jac_t(w, x)[0]

# compare with numpy
print(((1 - np.tanh(x.dot(w)) ** 2) * w).T)
[[ 0.41997434  0.          0.41997434  0.          0.        ]
 [ 0.          1.          1.          0.          0.        ]
 [ 0.          0.          1.          0.          0.        ]]
[[ 0.41997434  0.          0.41997434  0.          0.        ]
 [ 0.          1.          1.          0.          0.        ]
 [ 0.          0.          1.          0.          0.        ]]

Note that we need to iterate over the indices of y and not over the elements of y. The reason is that scan create a placeholder variable for its internal function and this placeholder variable does not have the same dependencies than the variables that will replace it.

Scan Example: Accumulate number of loop during a scan

import theano
import theano.tensor as T
import numpy as np

# define shared variables
k = theano.shared(0)
n_sym = T.iscalar("n_sym")

results, updates = theano.scan(lambda:{k:(k + 1)}, n_steps=n_sym)
accumulator = theano.function([n_sym], [], updates=updates, allow_input_downcast=True)

k.get_value()
accumulator(5)
k.get_value()

Scan Example: Computing tanh(v.dot(W) + b) * d where d is binomial

import theano
import theano.tensor as T
import numpy as np

# define tensor variables
X = T.matrix("X")
W = T.matrix("W")
b_sym = T.vector("b_sym")

# define shared random stream
trng = T.shared_randomstreams.RandomStreams(1234)
d=trng.binomial(size=W[1].shape)

results, updates = theano.scan(lambda v: T.tanh(T.dot(v, W) + b_sym) * d, sequences=X)
compute_with_bnoise = theano.function(inputs=[X, W, b_sym], outputs=[results],
                          updates=updates, allow_input_downcast=True)
x = np.eye(10, 2, dtype=theano.config.floatX)
w = np.ones((2, 2), dtype=theano.config.floatX)
b = np.ones((2), dtype=theano.config.floatX)

print(compute_with_bnoise(x, w, b))
[array([[ 0.96402758,  0.        ],
       [ 0.        ,  0.96402758],
       [ 0.        ,  0.        ],
       [ 0.76159416,  0.76159416],
       [ 0.76159416,  0.        ],
       [ 0.        ,  0.76159416],
       [ 0.        ,  0.76159416],
       [ 0.        ,  0.76159416],
       [ 0.        ,  0.        ],
       [ 0.76159416,  0.76159416]])]

Note that if you want to use a random variable d that will not be updated through scan loops, you should pass this variable as a non_sequences arguments.

Scan Example: Computing pow(A, k)

import theano
import theano.tensor as T
theano.config.warn.subtensor_merge_bug = False

k = T.iscalar("k")
A = T.vector("A")

def inner_fct(prior_result, B):
    return prior_result * B

# Symbolic description of the result
result, updates = theano.scan(fn=inner_fct,
                            outputs_info=T.ones_like(A),
                            non_sequences=A, n_steps=k)

# Scan has provided us with A ** 1 through A ** k.  Keep only the last
# value. Scan notices this and does not waste memory saving them.
final_result = result[-1]

power = theano.function(inputs=[A, k], outputs=final_result,
                      updates=updates)

print(power(range(10), 2))
[  0.   1.   4.   9.  16.  25.  36.  49.  64.  81.]

Scan Example: Calculating a Polynomial

import numpy
import theano
import theano.tensor as T
theano.config.warn.subtensor_merge_bug = False

coefficients = theano.tensor.vector("coefficients")
x = T.scalar("x")
max_coefficients_supported = 10000

# Generate the components of the polynomial
full_range=theano.tensor.arange(max_coefficients_supported)
components, updates = theano.scan(fn=lambda coeff, power, free_var:
                                   coeff * (free_var ** power),
                                outputs_info=None,
                                sequences=[coefficients, full_range],
                                non_sequences=x)

polynomial = components.sum()
calculate_polynomial = theano.function(inputs=[coefficients, x],
                                     outputs=polynomial)

test_coeff = numpy.asarray([1, 0, 2], dtype=numpy.float32)
print calculate_polynomial(test_coeff, 3)
19.0

Exercise

Run both examples.

Modify and execute the polynomial example to have the reduction done by scan.

Solution







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